An exhaust gas from diesel engines, etc. contains a large amount of carbon-based particulate matter, which adversely affects humans and environment when discharged into the air. Accordingly, ceramic honeycomb filters are used to remove particulate matter in exhaust gas parts connected to diesel engines, etc. As shown in FIG. 1, a ceramic honeycomb filter 10 comprises a ceramic honeycomb structure 11 comprising porous cell walls 3 defining large numbers of flow paths 2a, 2b and a peripheral wall 1, and plugs 4, 5 alternately sealing both ends 7, 8 of flow paths 2a, 2b in a checkerboard pattern.
A ceramic honeycomb filter with such a structure can be produced by the method described, for instance, in JP 2001-300922 A. JP 2001-300922 A discloses a method for producing a ceramic honeycomb structure comprising filling a plugging slurry to predetermined cells of an unsintered ceramic honeycomb molding at both ends, and sintering it. However, when the unsintered ceramic honeycomb molding is provided with plugs by the method described in JP 2001-300922 A, the ceramic honeycomb molding usually solidified by a water-soluble binder such as methylcellulose, etc. is brought into contact with a plugging slurry comprising a liquid component such as water, etc., so that water in the slurry intrudes into the molding. This softens the water-soluble binder in the molding, deforming the ceramic honeycomb molding, removes water from the plugging slurry, turning the plugging slurry less flowable, and fails to provide plugs with sufficient length.
JP 57-7215 A discloses a method for forming plugs in a sintered cordierite honeycomb structure, comprising attaching a film to each end surface of the sintered cordierite honeycomb structure, providing the film with holes at positions corresponding to flow paths in which plugs are formed in a checkerboard pattern, charging a cordierite-based plugging material into the flow paths through the holes of the film, and sintering the plugging material at the highest temperature of 1400° C. for 2 hours. Because the plug-forming method of JP 57-7215 A charges the plugging slurry into the sintered ceramic honeycomb structure, it is free from the problems of the method of JP 2001-300922 A that the ceramic honeycomb molding is deformed, and that the resultant plugs do not have sufficient length. However, the plug-forming method of JP 57-7215 A suffers insufficient bonding of plugs to cell walls due to the difference in sintering shrinkage between the sintered body and the unsintered plugs, and insufficient bonding strength. Namely, when an unsintered plugging material is charged into the flow paths of a sintered ceramic honeycomb structure and sintered, shrinkage and expansion substantially do not occur in the sintered body which has already undergone sintering shrinkage, but the plugs undergoes dimensional change such as shrinkage and expansion by a sintering reaction, resulting in gaps between the cell walls and the plugs and thus extremely low bonding strength between the plugs and the cell walls. Particularly when the plugs are formed by cordierite, they suffer large sintering shrinkage, making it likely that the plugs have low bonding strength to the cell walls.
JP 63-28875 B discloses a method for forming plugs in a cordierite-based ceramic honeycomb structure, comprising sealing the flow paths of a cordierite honeycomb structure obtained by extruding, drying and sintering at 1420° C. for 2 hours with a cordierite-forming material such as α-alumina, calcined talc, calcined kaolin, quartz or amorphous silica, mullite, etc., which is not reactive in a low-temperature region during sintering, in a checkerboard pattern on one end side, similarly sealing the remaining flow paths on the other end side in a checkerboard pattern, drying the plugging material at 150° C. for 2 hours, and sintering the plugging material at the highest temperature of 1400° C. for 6 hours. JP 63-28875 B describes that these cordierite-forming materials suppress the shrinkage of aluminum hydroxide, untreated clay, talc, etc., which are also cordierite-forming materials, at relatively low temperatures, and cause a cordierite-forming reaction drastically at temperature of 1200° C. or higher, resulting in volume expansion, which is effective to reduce the sintering shrinkage. However, the method of JP 63-28875 B cannot completely suppress the shrinkage of the plugging material even though it is made of a shrinkage-suppressing, cordierite-forming material. It is thus likely that there is not sufficient bonding strength between the plugs and the cell walls because of insufficient bonding of the plugging material to the cell walls. Particularly in the case of a sintered cordierite honeycomb having an outer diameter of 150 mm or more, the plugging slurry introduced into flow paths may not form plugs of sufficient length depending on the viscosity of the slurry. When the plugs have insufficient length, the shrinkage of the plugging material causes insufficient bonding of the plugging material to cell walls, resulting in insufficient bonding strength between the plugs and the cell walls.
When plugs are formed and sintered in the sintered ceramic honeycomb structure to provide a ceramic honeycomb filter, two sintering steps; a step of sintering the ceramic honeycomb structure, and a step of sintering the plugging material to fix it to the cell walls of the ceramic honeycomb structure, are needed, resulting in a longer production time and lower productivity, leading to higher cost. As a result, it has been desired to shorten the sintering time.